It is often desirable or necessary to temporary isolate selected sections of a pipeline while the pipeline system is in operation, when performing maintenance, branching, repair, etc. Such isolation is commonly performed by plugs which are positioned and set at the desired location, allowing downstream operations to continue unimpeded. Using a plug which is inserted into the pipeline, guided to the desired location and then set, is a procedure which is cost effective, flexible and possible in most pipeline configurations.
The plugs may be run into the pipeline with an umbilical or be autonomous plugs that are pigged and/or moved by tractor units within the pipeline. An autonomous plug may for instance be used in long subsea pipelines for sealing these off at the desired site, and thereafter removed from the pipeline when the work is has been completed. Some plugs are set by extending slips into contact with the pipe wall and forcing a sealing element against the pipe wall. Some plugs are designed to self-lock into the set positions as long as there is a pressure difference across the plug. One example of an autonomous plug is disclosed in WO 2008079016.
Plugs of this type, which may be manipulated between set and unset states, are suitable for applications where the pressure differences across the plug are comparably high, for example up to about 150 bar.
In applications where the pressure differences are comparably low—for example around 5 bar, it is often more convenient to use friction pigs (also referred to as “high-friction pigs”) for pipeline isolation. Friction pigs are normally pigged to the desired location, whereupon the differential pressure is removed, allowing the friction pig to come to rest in the pipeline. These types of pigs have typically no moving parts, but their sealing elements comprise a resilient material with certain stiffness. One example of a suitable resilient material is polyurethane. The sealing function of a friction pig thus relies entirely on the friction between the pipeline wall and the pig material.
It is well known in the art to provide a friction pig with a transponder in order to track its travel through a pipeline. One such tracking system is the SMARTTRACK™ tracking system (TDW Delaware, Inc.), developed by the present applicant. This system allows two-way electromagnetic communication between the transponder mounted on the pig (or a plug) and a transceiver outside the pipeline. Each transponder is given a unique identification number, which is communicated to the transceiver and, if required, further to a control unit (a PC or similar). Thus, when the transponder is in an active mode, it sends an electromagnetic signal, enabling an operator using the transceiver to track the pig's travel through the pipeline, and e.g. to determine when the pig has reached a pre-determined location in the pipeline.
As described above, once a friction pig has been placed at its intended position in order to isolate a section of a pipeline, its sealing function relies entirely on the friction between the pipeline wall and the pig material. A pig of this type has no inherent fail-safe properties. An operator performing work on the isolated section of the pipeline must have confidence in the pig's sealing capabilities, and it is therefore of great importance that there are no leaks from one side of the pig to the other and that the pig remains more or less stationary at the intended position.
It is therefore a need for a system and a method of monitoring such pig.